Group 14: Carbon, Silicon, Germanium, Tin and Lead
Group 14 Elements
The group 14 elements (Carbon, Silicon, Germanium, Tin and Lead) display a remarkably wide variety of characteristics. The properties of the elements vary so much because the metalloid line transverses the column; nonmetals, metalloids and metals are all represented in this group. Carbon exhibits the uniqueness principle and has many properties that foster life. The metalloids (Silicon, Germanium) are used for semiconductors, and the metals (Tin, Lead) have their own properties and uses.
Perhaps the most extensively studied element in the periodic table, Carbon is the first row element in Group 14. It has been known since ancient times, yet Lavoisier is credited with identifying Carbon as an element.(388) Carbon is so diverse chemistry is divided into organic and inorganic chemistry. All living organisms are built from carbon compounds, yet carbon exhibits a variety of chemistry outside of living specimens.
Allotropes of Carbon
Carbon has two naturally existing allotropes and a third discovered by scientists in the 1980s. The two naturally occurring allotropes are graphite and diamond. Graphite is formed by sheets of Carbon bonded through pπ- pπ bonds. Only London forces hold the sheets of Carbon together, so that is why graphite is such a soft compound. Diamond, on the other hand, is one of the hardest known materials. Diamonds are held together by sp3 hybridized C-C bonds, which are a lot stronger than pπ bonds. The third allotrope of Carbon is fullerene, discovered by Kroto et al in 1985.(400) The fullerenes form “buckyballs” and nanotubes. The balls, which come in a variety of sizes, are described in essence as “a section of graphite curled up into a ball.”(401) These balls are incredibly stable because of symmetry and π electrons available. Scientists have experimented placing atoms and molecules inside of a buckyball. Carbon atoms can also form nanotubes, which are bonded the same way as in buckyballs but are long tubes. These tubes are flexible and stronger than steel at a fraction of the weight.(404)
Silicon has also been known for centuries, but was first isolated and purified in 1824.(390) It is naturally found in sand as a silicate, and glass is also composed of SiO2. In modern uses Silicon is used as a semiconductor. Silicon and the larger group 14 elements exhibit pπ- dπ bonding, something Carbon cannot do.
Germanium was discovered as part of a silver ore in 1886. It was used by Bell Lab as a semiconductor in their first transistors, yet Silicon is a more popular choice for semiconductors today.(390)
Tin is one of the two metals in Group 14 and it has been used throughout history in bronze and other alloys. It has the most stable isotopes of any element, and two allotropes that can interconvert. Β-Tin is the form used in industry for various products. It is hard and white, yet when exposed to temperatures below 13.2 degrees Celsius for extended periods of time it converts to α-Tin, which is frail and crumbles.
Lead has been known since ancient times; with its first uses being in pipes and cups. It was used in paints until recent times, and is used for batteries. Despite Lead’s many uses throughout history it is toxic to humans. The heavy metal remains in the body for quite some time, and accumulation can result in anemia, fatigue, headache, weight loss and constipation.(409)
Chemistry of Group 14 Elements
Catenation is the ability of an element to bond to another atom of the same element. Carbon atoms form very strong bonds to each other and are thus able to form stable chains. The C-C bond energy is 356 kJ/mol and Silicon’s is much weaker at 226 kJ/mol. The reason for the strength is Carbon’s small size allows for close bond lengths and more efficient sp^3 orbital overlap. Si-Si bonds do not usually occur because the Si-O bond is more stable through pπ- dπ bonding, which will be explained later.
C-H bonds are pretty stable, so the reactions of “Carbon hydrides” (alkanes) do not readily occur. Rogers notes how a flame or catalyst is required for methane to react with oxygen. Silates do not require catalysts to react with Oxygen, as the Si-O bond is more stable than the Si-H bond.
- SiH4(g) + 2O2(g) ---> SiO2 (s) + 2H2O (g)
The hydrides are formed with the empirical formula AnH2n+2. Carbon can form huge chains, while Si and Ge can make it up to N=10. Tin has only been formed for N=2, and Lead hydrides are too unstable.
When group 14 elements bond with oxygen they form acidic anhydrides. Carbon, exhibiting the uniqueness principle, forms carbon dioxide. CO2 is a gas at room temperature, whereas the analogous group 14 oxides are solids. Tin follows the trend of forming a dioxide, but lead’s more stable oxidation number is 2, so PbO is more prevalent.
Carbon readily forms bonds with halides. Individual carbon atoms form tetrahalides CX4, or chains of -CX2- can form. Silicon also forms tetrahalides and chains, but through d orbital bonding silicon tetrahalides can react with other compounds. For germanium, tin and lead dihalides and tetrahalides exist. Germanium tetrahalides are more stable than dihalides, but dihalides can be prepared through by reacting a tetrahalide with elemental germanium. Tin and lead almost exclusively form dihalides.
- GeF4 (s) +Ge(s) --> 2GeF2
- SnO (s) + excess HF --> SnF2 +H2O
One of the major differences is carbon chemistry and silicon chemistry is silicon (and other third row elements) has empty d orbitals. Silicon has a high effective nuclear charge, with its nucleus pulling orbitals tightly. The d orbitals then have approximately the same size as p orbitals, and can accept electrons. The ability for silicon to accept electrons from oxygen allows the Si-O bond to be relatively strong. A classical example of the pπ- dπ interaction is the silicon analog to ammonia. Ammonia has a lone pair, acts as a Lewis Acid and has a pyramidal geometry. The silicon d orbitals in the trisilyl compound accept the lone pair on the nitrogen. The trisilylamine compound is unable to react in acid/base reactions and it has a planar geometry since there is no lone pair repulsion.
Carbon-14 dating is a well-known process to determine approximately how old a fossil is. Carbon-14 naturally occurs and is formed by neutrons colliding with nitrogen. The end product of this process is a carbon-14 atom and a deuterium. The radioactive carbon isotope decays back to a nitrogen atom. Since the properties of carbon-14 are essentially the same as carbon-12, carbon-14 is used by living organisms in their biochemical processes. When the organism dies it cannot excrete the carbon-14, and the carbon-14 present begins to decay. Scientists can relate the age of the specimen to the concentration of carbon-14 still remaining in the sample.
Another dating technique is measuring the different lead isotopes in a sample. Lead can occur naturally as lead-204, or result from the decay of uranium, actinium or thorium. Uranium decay results in lead-206, actinium results in lead-207 and thorium results in lead-208. By analyzing the concentrations of each lead isotope (the 3 radioactive isotopes have half-lives on the order of billions of years) the relative age of cosmic bodies can be determined.
When silicon and germanium form their covalent network analog to diamond, some electrons are free to move throughout the network because of the lower effective nuclear charge. Free electrons produce a current through the material, and as more energy is applied to the material more electrons are freed and conductivity increases.
The band theory explains how semiconductors conduct electricity. When excited with heat or light covalently bonded electrons “jump” from the valence band to the conduction band. The size of the gap between the two bands indicates how well the material conducts; a smaller energy gap allows electrons to jump easier and conduct better. Electrons leave holes in the network when they jump to the conduction band, so when a potential difference is applied to the semiconductor electrons in the valence band are able to move and fill the holes. A common practice to increase conductivity of semiconductors is doping the element. By adding trace amounts of an extra group 13 element (B or Ga) an extra hole is formed because group 13 elements have 1 less electron. Likewise, adding trace amounts of group 15 (P or As) adds another electron to the conduction band.
Semiconductors are important because their conductivity increases with temperature. Computers and other electrical devices generate heat, so a semiconductor is able to continue conducting a current whereas a metal’s ability to conduct decreases.
The chemical definition of glass is "a homogeneous, noncrystalline state resembling that of a liquid whose rate of flow is so slow that it appears to be rigid over long periods of time." (418) This state results from melting silica, but the bonds reform irregularly. This irregular lattice can slowly move, but appears to be stationary.
Glasses are predominately made of SiO2. Other elements added to the glass structure produce particular effects, such as strontium oxide glass absorbing X rays. Metals are added for color, and some compounds are embedded in the glass that can react. For example, AgCl decomposes when exposed to light, darkening the glass.
Rodgers, Glen E. Descriptive Inorganic, Coordination, and Solid-State Chemistry. Canada: Thomas Learning, 2002.